(1) Field of the Invention:
The present invention relates to a pneumatic tire having an improved tread, and more particularly relates to a steel cord-reinforced pneumatic tire for truck and bus having a tread having improved cut resistance, chipping resistance and wear resistance without deteriorating the resistance against heat build-up during the travelling not only on paved road but also on off road.
(2) Description of the Prior Art:
Recently pneumatic tires for truck and bus have become large in size and travel on road at a higher speed, so that a pneumatic tire having a reinforcing layer containing steel cords embedded therein has often been used. However, such reinforcing layers are highly rigid, and when the tire travels on both paved roads and off roads where stones and rocks are scattered thereon, the tread is apt to be separated due to the heat generated at high speed running and under the heavy load before the tread is completely worn. Further it is apt to be subjected to cut and chipping when the tire rides on the sharp stones and rocks. Therefore, the tread rubber of tire for truck and bus is required to have the resistance against heat build-up, cut resistance and chipping resistance in addition to the wear resistance of the tread rubber itself, and the development of rubber composition for tread having the above described four properties in a well-balanced state has been commercially demanded.
Natural rubber is generally poor in the wear resistance but is high in the cut resistance and chipping resistance, and attempts to improve the wear resistance of natural rubber have been made by compounding a large amount of carbon black having a small particle size and having a high reinforcing property to natural rubber. However, when a large amount of carbon black is compounded to natural rubber, a large amount of softening agent must also be used, and therefore the resulting rubber composition is poor in the resistance against heat build-up, workability, cut resistance and chipping resistance. Hence, a satisfactory rubber composition for tread has not yet been obtained by the use of natural rubber.
While styrene-butadiene rubber has high cut resistance, chipping resistance and wear resistance. It is poor in the resistance against heat build-up, and moreover requires a large amount of softening agent in the processing. Therefore, the resistance against heat build-up of styrene-butadiene rubber is noticeably lowered, and the tread using the styrene-butadiene rubber composition separates, and a satisfactory rubber composition for tread has not yet been obtained by the use of styrene-butadiene rubber.
Reinforcing methods by the use of commonly used carbon black cannot improve both of chipping resistance and wear resistance of rubber. For example, when carbon black, such as ISAF carbon black, which improves the wear resistance of rubber, is compounded to rubber, the chipping resistance of the rubber lowers. When carbon black, such as LS-ISAF carbon black, which improves the chipping resistance of rubber, is compounded to rubber, the wear resistance of the rubber lowers. Therefore, a tread rubber composition suitable for steel cord-reinforced pneumatic tire for truck and bus, which travels on both of paved road and off road, has not yet been obtained.